Magnetorheological fluids are magnetic field responsive fluids containing a field polarizable particle component and a liquid carrier component. These rheological fluids (also referred to herein as controllable fluids) are typically used within the working gap of devices such as dampers, shock absorbers, elastomeric mounts, clutches, brakes, and valves to provide varying damping forces, and are activated by the application of a magnetic field.
With regard to the composition of magnetorheological fluids, the particle component of a magnetorheological fluid must comprise a particle or powder that exhibits magnetic field polarizability, and the carrier component can be any oil or liquid medium known in the art, such as silicone oils, silicone copolymers, mineral oils, synthetic hydrocarbons, perfluorinated polyethers, polyesters and the like. Fluid compositions which undergo a change in yield strength of apparent viscosity in the presence of a magnetic field are commonly referred to as Bingham magnetic fluids or magnetorheological materials. Magnetorheological materials normally are comprised of ferromagnetic or paramagnetic particles typically greater than 0.1 microns in diameter, dispersed within a carrier fluid. In the presence of a magnetic field, the particles become polarized and are thereby organized into chains of particles within the fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall materials and in the absence of a magnetic field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the overall materials is correspondingly reduced.
Desirably, when exposed to a magnetic field the controllable fluids are designed to increase the force output of the device containing the controllable fluid. "Force output" as used herein means the damping force, torque, braking force or similar force depending upon the device. Generally, an increase in the particle amount of the controllable fluid results in an increase in the maximum yield strength of the fluid and, consequently, an increase in the maximum force output of the device. Unfortunately, the real or off-state viscosity of the controllable fluids in the absence of an applied magnetic field increases with increased particle amount. The particle amount, thus, is limited to a level consistent with the real or off-state viscosity requirements for the controllable fluids when the controllable fluids are not subject to a magnetic field.
U.S. Pat. No. 2,575,360 discloses an electromechanically controllable torque-applying device that uses a magnetorheological material to provide a drive connection between two independently rotating components, such as those found in clutches and brakes. A fluid composition satisfactory for this application is stated to consist of 50% by volume of a soft iron dust, commonly referred to as "carbonyl iron powder" (8 microns average size), dispersed in a suitable liquid medium such as a light lubricating oil.
U.S. Pat. No. 4,992,190 and U.S. Pat. No. 5,167,850 disclose rheological materials that are responsive to a magnetic field. The composition of these materials is disclosed to be magnetizable particles and either silica gel or carbon fibers dispersed in a liquid carrier vehicle. The magnetizable particles can be powdered magnetitite or carbonyl iron powders with insulated reduced carbonyl iron powder, such as that manufactured by GAF Corporation, being specifically preferred. The carbonyl iron particles are further described to have a particle size in the range of 3 to 6 microns.
U.S. Pat. No. 2,751,352 and Australian Patent Specification 162,371 disclose magnetorheological fluids wherein the finely divided magnetizable particles are inhibited from precipitating or settling out of the fluid system. The inhibition of particle settling is accomplished by the addition of a minute amount of an oleophobic material to the magnetic fluid. The particle component is further described to be any material possessing magnetic properties and having the dimensions of from 2 to 100 microns, preferably from 5 to 30 microns. Examples of particles include powder iron and iron oxide, with powder iron produced by the decomposition of iron pentacarbonyl being particularly useful.
U.S. Pat. No. 2,772,761 describes an electromagnetic clutch that utilizes a particulate composition that is prepared by mixing iron powder with colloidal graphite dispersed in a resin and then heating the mixture iron powder and colloidal graphite dispersion until the mixture is dry. The dry mixture is used in an electromagnetic clutch. The purpose of the dry mixture is to prevent packing of the iron powder in the electromagnetic clutch. There is no disclosure of a liquid formulation that includes a liquid carrier component.
U.S. Pat. No. 5,354,488 discloses an electrorheological magnetic fluid that consists essentially of a vehicle, magnetizable partilces suspended in the vehicle, and a dispersant. The dispersant is non-magnetizable particles that have no dimension greater than 10 nanometers.
An article entitled "The Magnetic Fluid Clutch" by Jacob Rabinow in AIEE Transactions, Vol. 67, pp. 1308-1315 (1948) mentions that higher magnetic permeabilities can be obtained by using "aggregates of large and small particles." There is no description of any specific aggregates.
A book entitled "Particle Packing Characteristics" by Randall M. German (Metal Powder Industries Federation, Princeton, N.J., ISBN 0-918404-83-5, 1989) describes particle packing phenomenon in general.
None of the patents or publications, however, show methods or formulations of magnetorheological fluids to increase the output of a device containing a magnetorheological fluid without causing an unacceptable increase or degree of variation in the real or off-state viscosity of the magnetorheological fluid.